Investigation of the effectiveness of CFRP strengthening of concrete made with recycled waste PET fine plastic aggregate

In recent decades, several studies have considered the use of plastic waste as a partial substitute for aggregate in green concrete. Such concrete has been limited to non-structural applications due to its low strength. This raises whether such concrete can be enhanced for use in some structural applications. This paper reports an attempt to develop a structural-grade concrete containing plastic waste aggregate with high proportions of substitution and confined with carbon fiber reinforced polymer (CFRP) fabrics. Experimental research was conducted involving the casting and testing 54 plain and confined concrete cylinders. A concrete mixture was designed in which the fine aggregate was partially replaced by polyethylene terephthalate (PET) waste plastic at ratios of 0%, 25%, and 50%, and with different w/c ratios of 0.40, 0.45, and 0.55. The results show that confinement has a substantial positive effect on the compressive behavior of PET concrete. The enhancement efficiency increases by 8–190%, with higher enhancement levels for higher substitution ratios. Adding one layer of CFRP fabric raises the ultimate strength of samples that have lost compressive strength to a level close to that of unconfined samples not containing PET. This confinement is accompanied by an increase in the slope of the stress-strain curve and greater axial and lateral strain values at failure. For the specimens confined by CFRP fabric, PET aggregate can be used as a partial substitute for sand at a replacement ratio of up to 50% by volume for structural applications. This paper also considers the ability of existing models to predict the strength of confined-PET concrete circular cross-sections by comparing model predictions with experimental results. The strength of confined PET concrete elements can’t be accurately predicted by any of the models that are already out there. It’s important to come up with a new model for these elements.


Introduction
Concrete is one of the world's most popular and widely-used construction materials [1][2][3]. Every year, around 12 billion tons of concrete are produced worldwide. The ongoing boom in the construction sector has resulted in increased demand for building materials like cement and aggregate. However, aggregate is a non-renewable resource. Continuous quarrying has negative environmental consequences and ultimately depletes aggregate availability. Therefore, measures to reduce the demand for the aggregate need to be developed. On the other hand, PET waste is a form of plastic waste that is growing in lockstep with human waste. PET is one of the major types of plastic and a member of the thermoplastic polyester family [4][5][6][7]. The main issue with plastic waste is that it can contain organic and inorganic components, such as food waste; this complicates recycling, and much of this material ends up in landfills. As a result, there is likely to be a lack of landfill sites in the future and increased environmental impact because most wastes are non-biodegradable and stay in the environment for tens of thousands, if not hundreds of years [8][9][10][11]. Therefore, valorization of waste plastic as fibers [12,13] or concrete aggregates [6,14] has become an opportunity. For example, [15] studied developed the concept of a new preplaced aggregate fiber reinforced concrete (PAFRC) reinforced with waste polypropylene (PP) carpet fibers and investigated its strength properties. Palm oil fuel ash (POFA) was used as a partial cement replacement. Six PAFRC mixes with fibers varying from 0 to 1.25% with a length of 30 mm were made by the gravity method. The study revealed that the carpet fibers have the potential to be used in PAFRC by developing their strength properties.
The principle of adding a substance to another has been used since ancient times to enhance the properties of composite materials. For example, horsehair and straw were added to clay to enhance brick characteristics [16,17]. Furthermore, concrete has been used with weaker materials to achieve composites with the necessary mechanical properties [18][19][20]. This includes the potential for turning plastic waste into construction materials by recycling it into green concrete [21,22]. As a result, the recycling rate will improve, and demand for natural raw material production will decrease. In this way, the environmental pressure on the concrete sector could be reduced, eliminating the need for natural capital and contributing to sustainable production [23,24]. For this purpose, in recent decades, several studies have considered waste plastic as a substitute aggregate in green concrete (also known as eco-friendly concrete) [25][26][27][28]. This approach has been affirmed by many studies which have argued that such recycling is essential for the ecosystem and economic gain [18][19][20]29].
The use of PET as a potential alternative to aggregate in concrete will not lead to the concrete being polluted, but some characteristics of the concrete may be affected [22,30]. In most instances, plastic wastes are used as coarse or fine aggregates in concrete. In previous investigations, specific techniques were used, such as chipping machines or hand cutting, to transform the material into a form suitable for addition to concrete mixes. Generally, different plastic additives have different effects on concrete properties [31,32]. Therefore, many studies have been carried out over the last three decades to study the effect of plastic waste on concrete [33][34][35][36][37][38][39][40][41][42][43][44][45][46][47][48]. However, there are still some negative issues that previous studies have not addressed or solved, such as the decrease in overall mechanical properties when replacing natural aggregates with plastic waste. Most importantly, past studies have indicated that concrete utilizing plastic waste as aggregate is likely to be only applicable to non-structural applications due to its low strength.
In contrast, throughout the last four decades, research has been conducted on the impact of FRP wrapping on the strength and ductility of wrapped concrete under various types of wrapping and loading conditions, with the corresponding development of experimental and design-oriented models . Most investigations have been carried out on cylindrical specimens wrapped in various types of FRP composites, which have no steel reinforcement. Such studies have shown that circular cross-sections have the most effective confinement, whereas square and rectangular sections have the least effective confinement. More confinement can be achieved by wrapping additional layers around the square or rectangular sections when increasing the rounding of corners is difficult. However, a thorough review of the literature found that no study has been done yet to see how well CFRP wrapping concrete made from PET waste works.
Generally, concrete containing PET can be used for non-structural purposes that do not require high compressive strength. However, there seems to have been no attempt to transform PET concrete into concrete capable of being used in structural applications. One way this might be achieved is by wrapping PET concrete with CFRP, and the purpose of this paper is to describe an investigation into this matter. The PET concrete considered in this paper is of a type where PET material has been added to replace a proportion of the aggregate. This work reported in this paper includes an experimental program and an evaluation of whether the design-orientated models reported in the literature for normal and high-strength of concrete are also applicable to confined PET concrete.

Significance of the study
The use of renewable materials has recently been observed in many sectors for economic and environmental reasons, in which the utilization of recycled plastic is a significant step toward sustainability. On the other hand, as is well known, FRP reinforcement is used to advance the mechanical properties of the concrete member and structural performance, but little is known about the effect of confining concrete that contains plastic waste. Therefore, the uniqueness of this study is that the behavior of concrete containing PET plastic waste confined by CFRP fabrics has not been investigated yet. This study will attempt to bridge this gap.

Materials
In this test program, ordinary Portland cement (OPC) Type I, with the brand name Tasluja, was used. The chemical properties and physical properties of the OPC are presented in Tables  1 and 2, respectively. Natural sand from the Khabour quarry in Duhok city was used in the concrete mixes. The grading test and physical properties of fine aggregate are presented in Table 3. Furthermore, crushed natural aggregate from the Sejie zone in Duhok city was used to prepare mixes, with the nominal maximum size passing through a 19 mm sieve. The gravel was cleaned and washed with water several times and allowed to dry in the air. Generally, water suitable for drinking is also suitable for use in concrete. In all concrete mixes and for curing of specimens, potable tap water at laboratory temperature without salt or chemicals was used. To improve workability, a high-range water-reducing admixture (superplasticizer) known as Sika1 ViscoCrete1-1316 Hi-Tech was added to the mixes. The manufacturer recommends that the dosage should be in the range of 500-1500 gm for 100 kg of cement. In addition, this type of admixture is compatible with ASTM C494 (types D and G) [94]. Table 4 shows the key properties of this superplasticizer.
Furthermore, in this investigation, PET particles were prepared by grinding PET waste bottles (type BC210) [95]. These PET bottles were supplied by the Light Plastic Factory [96]. The PET waste particles were produced in the following steps: 2. Shred and grind the bottles to a size similar to sand using a plastic granulator machine (SG-600F Model SML). This machine is used for plastic manufacturing by the Light Plastic Factory.
3. Sort the particles using sieves, and retain particles that pass through a 4.75 mm sieve. See After the PET aggregate was prepared, it was evaluated in grading by sieve analysis, as illustrated in Table 5. The physical and mechanical characteristics of the PET material are shown in Table 6 as provided by the Light Plastic Factory [96]. Due to the plastic texture and the plastic particle types, which are often flaky, angular, and irregular particles, the sieve analysis of PET aggregate does not conform to that of natural sand grading, as the fine natural aggregate is typically composed of spherical and granular particles.
Used in this test program were unidirectional CFRP sheets (SikaWrap-300C) [97] with fibers directed along the longitudinal axis. The CFRP sheet characteristics depend on the specifications offered by the supplier, Sika Company, and are shown in Table 7. Epoxy resins are generally utilized to bond CFRP to concrete. The adhesive material Sikadur-330 [98] was used in this test program. Five CFRP coupons with an average dimension of 15 mm × 250 mm and  [99]. The test data on CFRP coupons are presented in Table 7. The epoxy resin adhesive system consists of the main resin portion (Part A, white color) and the hardener (Part B, grey color), blended at a particular volume ratio of 4A:1B for about 10 minutes until the color becomes grey. It is then applied to the concrete surface using a paintbrush. A table called " Table 8" shows the material properties of an epoxy adhesive made by the company called "Sika."

Preparation and details of samples
In this experimental study, nine concrete mixes were produced containing different volumetric replacements of fine natural aggregate (0%, 25%, and 50%) by PET plastic waste with three different grades: M20, M30, and M40. The mix design was made following the American method ACI 211.1-91-R-02 [104]. A total of 54 cylinders with dimensions of 150 × 300 mm were prepared and tested (3 replacement ratios × 3 W/C ratios x wrapped/unwrapped × 3 repeats = 54). Three test specimens (i.e., three repeats) were considered for each case to ensure the reliability of the test results. These cylinders were divided into nine mixes (3 replacement ratios × 3 W/C ratios), with six cylinders in each mix.
To monitor and standardize the mixing process for all experiments, the mixing for all concretes was carried out in an electric rotary tilting drum mixer of 0.1 m 3 capacity by the procedure specified in ASTM C192/C192M [105]. A constant amount of 0.035 m 3 of materials was arranged for each mixture. Shovels and scoops were used to deposit the mixed concrete into the moulds. The same methodology was used for the preparation of all mixtures. After the mixing process was finished, the mixed concrete was poured into the iron moulds. The moulds were cleaned before casting, rigidly tightened, and lightly oiled to avoid adhesion to the concrete. After mixing, the moulds were filled and the concrete compacted by a Mallet hammer according to ASTM C192 [105]. Good quality concrete must be cured. For this reason, 24 hours after concrete casting, all specimens were put in a curing basin at around 25˚C. The curing status of the laboratory basin was adopted from ASTM C192 [105]. Fig 2 shows the preparation and curing process of the cylinders. Capping the concrete cylinders is significant to confirm that the load is uniformly distributed on the cylinder's surface during compression testing. For this purpose, before testing, all the concrete cylinders were capped with a 3 mm thick layer of sulfur capping compound. Capping the cylinders followed the procedures prescribed by ASTM C617 [106]. Moreover, tests were performed at the age of 90 days.

CFRP fabric confinement
Prior to wrapping, the 150 mm × 300 mm cylinders were dried and cleaned, and the concrete strength was 90 days age. At the beginning of the wrapping process, a thin layer of dust covering the specimens was removed with an air compressor. CFRP sheets were then cut into strips of the desired lengths and widths using scissors. Next, the epoxy coating was prepared by mixing the epoxy resin (parts A and B) in a proportion of 4A:1B. After the cylinders were placed upright, they were completely coated with epoxy using a paintbrush. The next stage was to wrap the CFRP sheets carefully around the cylindrical specimens, as shown in Fig 3. The fibers were aligned only in the hoop direction. A 120 to 125 mm overlap was provided to prevent  slippage between the CFRP layers. The location of the overlap for all specimens is shown in Fig  3. In addition, the upper and lower ends of the confined cylinders were further strengthened with 50 mm wide strips to prevent premature failure at the ends. Then, after 24 hrs., highstrength sulfur capping was applied to the top end of each specimen. Finally, the confined concrete specimens were rested in the laboratory for seven days.

Loading procedure
Fiber roving and uneven hardened epoxy needed to be smoothed to fix strain gauges on the cylinders. Sandpaper was used to smooth the fiber surface, which was then cleaned with isopropyl alcohol. Strain gauges were then installed at evenly spaced locations at the mid-height of all specimens. Two strain gauges (model PL-60-11-3LJC-F) were mounted for plain concrete, one horizontally and one vertically, in a T-shape. For the confined cylinders, four strain gauges (model BF350-3AA) were mounted, two horizontally and two vertically, to also form a T-shape. As shown in Fig 4, the load cell and strain gauges were connected to a data logger for data collection during compression. Compressive strength experiments were conducted on the concrete cylinder specimens following ASTM C39 [107]. The tests were performed using a universal test machine (Walter + Bai AG/ Switzerland) with a capacity of 3000 kN and a loading rate of 0.33 MPa/sec.

Results and discussions
The key test results at 90 days of curing of all 54 confined and unconfined specimens (cylinders with dimensions Ø 150 × 300 mm) are given in Table 9. The compressive strengths shown in

Effect of PET on strength reduction
The results shown in Table 9 demonstrate the impact of replacing a natural aggregate with a plastic aggregate. Generally, as the substitution percentage of PET particles increases, the compressive strength decreases. For example, compared to the reference mix, at 25% replacement (90 days   water is not absorbed by the PET, which does not participate in the water-cement reaction, causing poorer bonding and the creation of microscopic channels that can become pores after drying. Several authors have verified these observations [48,108,109]. Furthermore, an increase in the w/c ratio corresponds to a decrease in compressive strength, similar to conventional concrete mixtures. It is worth noting that at larger w/c ratios, the aggregate's coated surface is smaller, and as a result of the lower paste volume, the bleeding water content is higher. The excess water, which is primarily found around PET particles that do not participate in the water-cement reaction, causes a weaker bond between the cement paste and the PET particles and the formation of small channels that can form pores after drying, resulting in a reduction in strength.

Effect of CFRP wrapping on strength enhancement
The experimental results in Table 9 demonstrate the effect of wrapping concrete comprising plastic particles on the compressive strength performance of concrete after 90 days. Irrespective of the substitution ratio of PET and the w/c ratios, one layer of CFRP fabrics with full wrapping causes a substantial improvement of the ultimate compressive strength of PET-concrete cylinders compared to that of unwrapped cylinders. This strength increase can be described by the fact that confinement has served its purpose with PET concrete. Table 9 and Fig 5 also show that when the w/c ratio is reduced, the enhancement in strength efficiency decreases significantly. In other words, the effect of CFRP wrapping is more significant for samples with low compressive strength than for those with higher strength. The cause of this is that, for lower strength concrete, the concrete core can expand more, and, therefore, higher hoop strains can develop in the CFRP, providing greater confinement prior to rupture. As a result, it is noted that the efficiency of the strength enhancement increases significantly with the increase in the amount of substitution of PET aggregate.
Overall, the strength of cylinders containing PET aggregate and wrapped with one layer of CFRP fabric is significantly enhanced, as shown in

Stress-strain relationships
The stress-strain curves of the nine mixes of cylinders are presented in Fig 6, with the axial strain values being exposed on the left and the lateral strain values on the right. In general, the stress-strain relationships exhibit a linear portion, then as micro-cracking takes place, the shape of the curve becomes increasingly non-linear until it reaches the maximum stress. Fig 6 indicates that increasing the PET aggregate ratio for cylinders confined with CFRP fabrics leads to a significantly increased maximum strain. As the substitution ratio increases, there is a reduction in the initial slope of the axial stress-strain curve and in the value of stress at which the stress-strain curve ceases to be linear. Note that the slope of the non-linear part of the axial stress-strain curve is always positive, due to the confining pressure, which increases rapidly due to the rapid increase in lateral dilation of the concrete.

Failure modes.
The failure modes for some of the tested cylinders wrapped in CFRP are shown in Fig 7. It was observed that at low load intensities (initial load), an intermittent sound was heard due to microcracking in the concrete matrix. Several sounds were detected before the load reached its maximum level, at which such sounds were linked to the rupturing of fibers within the CFRP matrix. Finally, the CFRP sheets broke into rings with a high-intensity acoustic emission. Overall, all wrapped cylinders failed by the sudden rupture of the CFRP jacket close to the mid-height region outside the overlapping zone as the CFRP sheet suffered excessive tension in the hoop direction. It was also found that none of the CFRP- wrapped cylinders failed at the lap location, demonstrating reasonable adhesion and efficient load transfer between the concrete substrate and the CFRP. Two additional observations were made in the case of the CFRP-confined cylinders that contained plastic aggregate, especially at a high percentage of PET (50%), compared to their counterparts without PET: (i) the acoustic emission is less severe; and (ii) the tearing of the CFRP fabric is also less severe. These observations are thought to be due to the existence of plastic particles at the failure starting point, their high flexibility and elongated form, and the possibility that the plastic particles withstand a portion of the stress and act as a bridge between plastic particles parts.

Confinement action (a mechanism) of FRP
The passive confinement mechanism of the FRP shell on a concrete core occurs throughout compression. This action occurs as a consequence of the concrete core's hoop expanding under compression until the FRP ruptures [110][111][112]. The equivalent hoop strain and stress within the fabric increase as the axial stress increases, exerting restricting pressure on the core. In other words, under compression, the concrete core tends to expand (dilate) laterally, but the FRP fabric opposes this expansion, putting the concrete in a state of triaxial stress, resulting in a substantial

Lateral confinement pressure (f ι )
When a compression member is circumferentially wrapped with FRP composites, the fibers in the hoop direction respond against the circumferential concrete dilation. The concrete core is under even confinement pressure (expansion) [62,113]. This affords a hoop confining pressure (fι) which is directly affected by the CFRP wrapping and the cross-sectional area of the compression component. It is possible to compute the force equilibrium and radial displacement compatibility criteria between the concrete core and the CFRP fabric [82]. When the CFRP fabric's hoop strain exceeds its rupture strain, the specimen fails quickly in a brittle manner, achieving the CFRP's maximum confinement pressure (f ι,max ). Eq (1) could theoretically be used to calculate the value of f ι,max using the average axial strain at failure measured from tensile coupons. Also, such a value could be calculated based on the data provided by the manufacturer in combination with the average axial strain at failure measured from tensile CFRP coupon tests. However, as previously stated, this is likely to overstate fι, max .

PLOS ONE
Effectiveness of CFRP Strengthening of PET-Concrete

Effective lateral confinement pressure (fιe)
The CFRP fabric ruptures in the lateral direction as soon as the ultimate compressive strength of concrete samples confined by CFRP wraps is attained. Eq (2) can also be used to compute the effective confining pressure (f ιe ) using the recorded average lateral strain of CFRP-confined concrete from cylinder testing, as shown in Table 9. In this investigation, the average ultimate tensile strain captured at the mid-height of the coupon was 1.4 percent for a single CFRP ply, according to Table 7. When compared to the values in Table 9, it is obvious that the CFRP lateral rupture strain measured on concrete surfaces differs from the equivalent tensile strain measurements obtained from coupons. According to Lam and Teng [49], local deformation at cracks in the concrete surface, the presence of the overlapping zone, and the FRP composite curvature are reasons that explain this discrepancy.

Strength model
The use of transverse steel reinforcements, such as spiral or circular ties, increases the strength and ductility of concrete. Several models for concrete confinement with FRP have been developed since the 1980s. Most of these models were based on the regression of test data and were accomplished on plain concrete specimens. Regardless of their classification, most proposed active confinement relationships use the confinement model given by [114,115] based on tests of concrete samples confined with hydrostatic pressure. It was stated that the strength of confined concrete at failure, f 0 cc , could be presented as a linear function of the lateral confining pressure, f 0 cc , as given in Eq (3). In this equation, the strength ratio or confinement effectiveness is f´c c / f´c o , the confinement ratio is f ιe / f´c o and k 1 is a confinement effectiveness https://doi.org/10.1371/journal.pone.0269664.g008 coefficient. Table 10 evaluates existing strength models to predict the strength of the CFRP wrapped concrete cylinders tested in this work. The precision of 47 proposed models from the literature was assessed. For direct comparison, the predictions of all models, including those provided in codes and guidelines to predict f 0 cc have been supposed to potentially be adopted to the tested cylinders as part of this work. Note that the experimentally-measured strengths f 0 cc were mostly initiated to differ from those predicted by the previously published models. These differences may be due to the following reasons: (i) Foil gauges at mid-height were used to measure strain values under ultimate conditions. (ii) The majority of the models were established from experiments on plain samples made of various FRP composites, and (iii) the value of f ιe as labeled in Eq (2) was utilized rather than the f ι adopted in Eq (1). The foil gauges de-bonded in some cases prior to failure, so these data points aren't shown.

Evaluation of present strength models in prediction of f 0 cc
Of the models considered in Table 10, the models proposed by [68,75,78,80,116,117] seem to provide a prediction of f 0 cc which is closer to the control CFRP-confined test results declared in this paper. The models developed by [67,74,110,118] were found to offer the closest prediction of f 0 cc for all CFRP-confined cylinders containing PET waste established as part of this study. On the other hand, the models by [52,53] greatly overestimate the compressive strength in comparison with the results of the present investigation.
The results clearly show that confinement effectiveness is reduced with increased unconfined concrete strength. The confinement effectiveness of CFRP for concrete with a lower unwrapped compressive strength exhibits a higher confinement ratio than that for higher strength concrete. As the compressive strength increases, the stiffness of concrete also increases, resulting in less lateral expansion before fracture of the CFRP wrapping occurs. Therefore, the concrete experiences less confining pressure.
The predicted strengths of the confined concrete (Table 10) are compared to the test outcomes, as shown in Fig 9. This figure demonstrates the generally poor correlation of model predictions for PET concrete confined with CFRP fabric.

Conclusions
Concrete containing PET has been used for essentially non-structural purposes where the element can support its weight. To determine whether PET concrete can be used for some structural applications, the behavior of concrete containing PET aggregates and confined with CFRP fabric was studied in this investigation. The main conclusions arising from this study are as follows: 1. When CFRP-wrapped cylinders failed, the values of hoop strains at failure on the surface were often lower than those in flat coupons.
2. Based on laboratory findings, PET plastic aggregate may be used as a partial substitute for sand for structural purposes, with a ratio of up to 50% by volume, combined with CFRP confinement. As the substitution rate of PET particles increases, the compressive strength decreases. All samples confined with CFRP fabrics for all mixtures showed a significant  3. For cylinders confined with CFRP, as the replacement ratio increases, there is a decrease in the initial slope of the axial -strain curve and-strain curve and the value of stress at which the stress-strain curve ceases to be linear. Note that the slope of the non-linear part of the axial stress-strain curve is always positive due to the confining pressure, which increases rapidly due to the rapid increase in lateral dilation of the concrete. 4. The addition of a single layer of CFRP fabric wrap increased the ultimate load of samples to a level not less than that of unconfined samples without PET plastic waste. The recovery ratio ranged from 51% to 140%.
5. All of the samples that were confined failed because of the tensile failure of the CFRP fabric. The failure happened near the mid-height area outside of the overlapped area. 6. A comparison of the ultimate strength f 0 cc predicted by the range of confined concrete models found in the literature and test strengths was undertaken. It was noticed that these models do not offer a satisfactory prediction of the ultimate strength of PET concrete confined by CFRP fabric. However, the models confirmed that both confinement effectiveness (f´c c / f´c o ) and confinement ratio (f ιe / f´c o ) increase with increasing PET substitution.